The present invention relates to an improved distillation process for separating a hydrocarbon crude oil into its hydrocarbon product fractions.
The present invention further relates to an improved process for heat exchanging a crude oil with the hot circulating oil streams produced during the fractionation of the crude oil into its hydrocarbon product fractions.
In particular, the present invention relates to an improved process for separating a crude oil into its hydrocarbon product fractions utilizing a crude column and a vacuum column in combination with improved heat exchange and single or double stage flashing of the hot crude oil feed stock.
Those skilled in the art of hydrocarbon processing are familiar with the conventional manner in which a hydrocarbon crude oil is separated into its hydrocarbon fractions. Typically, the crude oil is heated in a series of heat exchangers wherein the crude oil picks up heat from circulating hot oil streams which are produced in a crude fractionating column and a vacuum column. The crude column fractionates the heated crude oil in the presence of stripping steam to produce a series of products of differing boiling range. The number of products produced in the crude column may range from two or three up to as many as eight, depending upon the type of crude oil being processed and the type of products which are desired by the refinery. Typically, the crude column will produce an atmospheric gas oil, a diesel oil, a kerosene, and a naphtha as side cut products. It will also produce a gasoline fraction as the overhead product and a reduced crude as the bottoms product. The reduced crude is sent to the vacuum column where it is separated into a light vacuum gas oil and a heavy vacuum gas oil as side cut products, and an asphalt fraction as the bottoms product.
Typically, the crude oil is brought up to the feed temperature required by the crude column by being passed through a series of hot oil heat exchangers and then being heated in a gas or oil fired heater. As an example, the crude oil will first be heated by a circulating naphtha stream which is produced by the crude column. Next the crude oil will be heated by the product kerosene which is leaving the crude column. It may be further heated by passing sequentially in exchange with the circulating kerosene stream and the diesel product stream. The hot crude oil may then be passed into a desalter which removes the salt and other water soluble contaminants from the hot crude. The desalted hot crude may be further heated by then picking up heat in another series of heat exchangers wherein the crude oil is sequentially heated by the circulating diesel oil stream, the atmospheric gas oil product stream, and the circulating atmospheric gas oil stream of the crude column. Next, the hot crude oil is sequentially heated in exchange with the circulating heavy gas oil stream and the asphalt product stream of the vacuum column. Finally, the hot crude oil is passed into the crude column heater wherein it picks up the final amount of required heat by means of convection and radiation produced through the combustion of gas or oil in the heater.
Since the heat exchange is conducted in series, a large pressure drop is experienced and it is typical for the discharge pressure of the crude oil pump to be from 125 to 200 psig. or even higher. This high crude oil pressure is a detriment to the crude oil fractionation process.
First of all, the high crude oil pressure causes the crude oil to be passed through the tubes of the heat exchanger bundles in the heat exchanger sequence since the crude oil is at a pressure which is higher than the pressure of the hot oil streams with which it is being heated. This is sound practice from an equipment design standpoint. The high pressure stream is put through the tubes since less steel is required in making the tube wall thickness of sufficient strength than would be required to make the heat exchanger shell thick enough to hold the same pressure. Thus, the heat exchanger costs are minimized. This is inefficient from a heat exchange point of view, however, since the cold crude oil should generally be put on the shell side of the heat exchangers for best heat transfer.
Furthermore, the high crude oil pressure can create a severe problem if a leak develops in the heat exchanger. Since the crude oil is at the higher pressure it will leak into the hot stream with which it is being exchanged. This will contaminate the hot stream with raw crude oil. If the hot stream is a hot oil circulating stream, the leakage will return contaminated hot oil to the crude column or vacuum column, and this contaminated hot oil stream will contaminate other product streams which are withdrawn from the column(s) at loci below the locus of the hot oil return. If the hot stream is a product stream, the leakage may cause operating problems at processing units elsewhere in the refinery where that product stream is being used as a feed stock, or if the product stream is passing to storage it will contaminate the product storage. In many instances, leakage into a product stream will necessitate shutting down the crude and vacuum columns in order to repair the leak. Shutting down the crude and vacuum columns may in turn cause the entire refinery to shut down.
Another disadvantage of the conventional crude oil heat exchanger train is that the entire system must be designed to withstand the highest vapor pressure which can be anticipated. The composition of the crude oil being fed to the crude unit will not be constant. The amount of light hydrocarbon contained in the crude unit feed will vary with the source of the crude oil. Thus the vapor pressure within the heat exchanger train, the desalter, and the crude column heater will vary as the amount of light hydrocarbon varies. Because of the uncertainties of crude oil composition, the heat exchange system must be overdesigned to be safe. For example, the desalter unit is normally designed to hold 150 psig. although the vapor pressure of the crude being desalted may be below 100 psig. for ninety percent of its on-stream time.